How many sheep your acreage can carry depends on some variables. The most obvious one is where you live, your local climate and the type of soil - these will all affect grass growth and its nutritional value, and whether or not your stock can live out all year round or need to be housed in winter. Heavy rainfall will tend to make the soil acid, which will lock up certain nutrients and reduce earthworm activity, for example.
The breed you keep will have an influence - you can fit in more little primitives than you can some larger types, and you can supplement their diet with browsing.
Your management system will affect your stocking rate in a couple of ways too. If you intend to make your own hay, for example, then you will have to shut up a certain acreage for a few months in spring, so that will be unavailable for grazing. This means that whereas if you buy in hay you might be able to keep a certain number of animals, but if you will be growing your own hay then you must keep fewer.
The other way reflects how you manage your grassland - whether you resow every few years, rotate your grass with other crops, or have permanent, species-rich pasture. How you replace nutrients will affect the nutritional value of the grass you grow - artificial fertilisers, manure or nothing.
So it's not just a case of a straight number per acre which covers everyone.